The present invention relates to a control system for a control drive of a double-acting hydraulic cylinder provided as a drive unit for a working tool of a processing machine by which a work piece, such as, for example, a steel plate, can be subjected to cold deformation actions such as punching or embossing.
A control system of the aforementioned type, used in conjunction with a hydraulic drive unit, is disclosed in, for example, unpublished German Patent Application P 37 35 123.0, wherein the drive unit includes a linear hydraulic cylinder constructed as a double-acting cylinder 1, with a double-diameter piston and dimensioned in such a way that a ratio F.sub.A /F.sub.G between a larger driving surface F.sub.A and a smaller surface or countersurface F.sub.G amounts to about 1/3. The fast forward movements of the hydraulic cylinder piston and the tool, movements by which the tool is brought towards the work piece and also caused to perform a part of the processing, are obtained by a forward motion cycle in which pressure is applied to both the surfaces F.sub.A and F.sub.G of the hydraulic cylinder piston, in the former case through a directional control valve, in the latter case through a pressure-controlled valve element of a surface-reversing valve. If the force thus developed by the piston in differential operation is insufficient to ensure, for example, the tool punching completely through the work piece, the surface-reversing valve, controlled by the pressure in the smaller driving pressure space of the hydraulic cylinder as soon as that pressure exceeds a certain threshold value, lying a preset amount below the maximum value of the output pressure of the pressure source, is switched into its position associated with forward movements under load. In this position the smaller driving pressure space of the hydraulic cylinder is relieved of pressure so that only the larger driving pressure space remains connected to the pressure output of the pressure source and becomes subject to a pressure of, for example, 200 bar. In order to avoid repeatedly switching the surface-reversing valve "to and fro" in cases where the required forward driving force is only slightly greater than the maximum force that can be obtained by differential operation of the cylinder, which, in unfavorable circumstances would not only retard the working process but could even lead to the piston stopping "dead" in a given position, the surface-reversing valve is constructed in such a manner that it will switch back to differential operation only after the required forward driving force has become smaller than the maximum forward driving force obtainable in differential operation of the hydraulic cylinder by an amount corresponding to a preset safety margin.
The hydraulic control system according to the above-noted German Application P 37 35 123.0 operates satisfactorily inasmuch as advantageously short working cycle times are obtained in numerous cases where the forward driving force, obtained by differential operation of the hydraulic cylinder, is more or less sufficient and the reversing valve is therefore obliged to switch to one-sided cylinder operation only in some very rare cases. If these short cycle times are to be duly exploited, which is particularly advantageous when one has to punch through a relatively thin sheet of steel, and sufficient power reserves are yet to remain available for processing thicker steel plates, a relatively high value must necessarily be chosen as the ratio between the two working surfaces of the hydraulic cylinder. But this means that once the surface-reversing valve has been excited, a correspondingly large increase in the maximum available forward driving force is obtained and this, even while the tool is still engaged in punching through, can lead to a considerable acceleration of the hydraulic cylinder piston, which will therefore have to be slowed down as soon a the reversing valve has switched back to differential operation of the cylinder. This process can lead to considerable jerking motion or action, and the jerking motion or action become all the more likely when the tool finds it relatively "easy" to punch through the work piece and thus facilitates the "bolting" of the driving cylinder before the piston can be slowed down by being switched back to differential operation.
It is quite true that one could think of attenuating the jerking motion or action by constructing the directional valve and or the movement control valve as an adjustable valve, say, some known type of follow-up adjusting valve. But this remedial measure by itself would not make any appreciable contribution to smoothing a working cycle of the type here described, not even if one were to use a follow-up adjusting valve with mechanical feedback of the actual value or position, because the regulating frequency of such a valve would still be relatively small as compared with the time interval in which these shocks can occur, so that the jerking motion or action would not be prevented in actual practice.
Likewise, the double-acting hydraulic cylinder, with one driving surface and one countersurface could possibly be replaced by another in which two driving surfaces are arranged in such a manner that the second can either reinforce or attenuate the forward driving force of the first and, controlling the pressure applied to these working surfaces by a follow-up adjusting valve. But this would call for an extremely costly design of the hydraulic cylinder itself and it would no longer be possible to use some standard unit and adapt such unit to the particular requirements of the case by adding an appropriate control peripheral equipment.
An object of the present invention resides in providing an improved control system of the aforementioned type which permits the control system to be used in conjunction with a simple double-acting cylinder as the driving unit of a hydraulic drive system and, notwithstanding a simple overall structure, to make it possible to obtain non-jerky operation of the system, if necessary even when the machine equipped with the drive has to be operated with a high-speed sequence of working cycles.
In accordance with advantageous features of the present invention, a hydraulic control system for a drive control of a double-acting hydraulic cylinder, provided as a drive unit of a processing machine in which a work piece, such as, for example, a steel plate is subjected to cold deformation, such as, for example, punching or embossing, is provided wherein a pressure source unit supplies pressures which can be made available at different pressure levels such as, for example, a lower pressure level, P.sub.N and a higher pressure level P.sub.H with an automatic switching to a required pressure level being achieved by a pressure-controlled reversing valve capable of a very fast switching operation. A switching of the surface-reversing valve takes place only after the pressure-controlled reversing valve has switched the pressure source unit either in a direction of the high pressure level P.sub.H or the lower pressure level P.sub.N. Additionally, with the above measures, a continuous adjustment in line with requirements of the operating pressure P.sub.H prevailing in a larger driving pressure space of the hydraulic cylinder by a follow-up adjusting valve operating with an electric signal or indication of a set value of in mechanical feedback of the actual value is effected.
By virtue of the above-noted features of the present invention, a substantially jerk-free and smooth sequence of working cycles is obtained, because with the above combination of measures, the follow-up adjusting valve, which, due to the aforementioned pressure switching, is effective with smaller displacements of through flow regulating valve elements so that the adjusting valve can act more "quickly" thereby making possible higher regulating frequencies which, in turn, are of a benefit in avoiding a jerking motion of the piston and tool. The control system of the present invention therefore insures comfortable and substantially noise-free operation of the machine even when fast working cycle sequences are being performed.
A "slowing down" of the piston of the driving cylinder during the final phase of the processing of the work piece will take place only after the pressure supply has been switched back to the lower pressure level P.sub.N, thereby greatly facilitating the slowing down process.
Bearing in mind that even the control system described in the aforementioned German Patent Application, depending on the purpose for which it is used, can be equipped with a follow-up adjusting valve as the directional control valve, possibly to ensure ready control of the execution of the movements when a CNC control is used, to realize a comparable control system in accordance with the present invention, the pressure source is constructed having two different output pressures and providing a pressure-reversing valve arrangement for permitting these different output pressure levels to be exploited as required. In technical terms, any additional cost incurred is rather small, so that the control system according to the invention can be considered as "simple" and the total cost of the driving and control system are not on the whole greatly increased by this additional technical effort, while a jerk-free operation of the machine is made possible. Moreover, the present invention reduces wear and tear and, consequently, the slightly greater investment costs have to be viewed against markedly smaller operating costs that greatly overcompensate the somewhat higher initial investments.
In accordance with the present invention, in a course of a processing cycle, the tool performs a fast forward movement towards the work piece, a working stroke in which the work piece is actually deformed, and a fast return movement to bring the tool back into a starting position for the next processing cycle.
In the present invention, the hydraulic cylinder has a total of two driving pressure spaces that are delimited in a mobile but pressure-type manner by different side surfaces F.sub.1 and F.sub.2 of a driving piston of the hydraulic cylinder, which driving piston is constructed as a double-diameter or differential piston.
By virtue of the provision of a double-diameter piston, it is possible, when driving and/or operating pressures derive from an output pressure of a pressure source are applied to both piston surfaces F.sub.1, F.sub.2, to control the feed and working movements of the tool in a fast forward operation. Additionally, when pressures applied only to a larger piston surface F.sub.1, while the smaller piston surface F.sub.2 is relieved of pressure, it is possible to control to forward working movements under a load that calls for a greater forward driving force.
Furthermore, by virtue of the double-diameter piston, when pressure is applied only to its smaller piston surface F.sub.2, while the large piston surface F.sub.1 is relieved of pressure, it is possible to control the fast return movements of the tool.
In accordance with further advantageous features of the present invention, an electrically-controlled directional control valve by which it is possible, upon switching the control valve into alternative operating positions to control the stroke and speed of the forward and return movements of the tool. In one of the operating positions of the control valve, pressure is applied to the driving pressure space of the hydraulic cylinder delimited by the larger piston surface F.sub.1, with the other operating position being associated with depressurizing the driving pressure space.
Advantageously, according to the present invention, a surface-reversing valve is provided and controlled by the pressure prevailing in the larger driving pressure space of the hydraulic cylinder, with the surface-reversing valve being adapted to be switched from an operating position associated with fast forward operation in which the pressure outlet of a pressure source is connected to the driving pressure space of the hydraulic cylinder delimited by the smaller piston surface F.sub.2, into an alternative position associated with the fast forward motion under a greater load in which the smaller driving pressure space of the hydraulic cylinder is relieved of pressure. By discharging the pressure from the larger driving pressure space F.sub.2 of the hydraulic cylinder, the surface-reversing valve can then be made to switch back into an operating position in which the smaller driving pressure space of the hydraulic cylinder is again connected to the pressure outlet of the pressure source.
The switching of the surface-reversing valve to a fast forward operation of the hydraulic cylinder under a load, in accordance with the present invention, will take place when the operating pressure i the larger driving pressure space of the hydraulic cylinder exceeds a value that corresponds to a large fraction, for example, 85% of the maximum obtainable operating pressure P.sub.H.
A subsequent switching of the surface-reversing valve into the operating position associated with the fast forward and return movements of the hydraulic cylinder takes place when the operating pressure prevailing in the larger driving pressure space of the hydraulic cylinder understeps a value that corresponds to a substantially smaller fraction, of for example, 30% to 50%, of the maximum exploitable operating pressure of the hydraulic cylinder.
A directional control valve of the present invention is constructed as a conventional follow-up adjusting valve which operates with an electronically controlled indication of a set value. The control is provided, for example, by a stepper motor with a mechanical feedback such as, for example, a worm gear, of the actual position making it possible to obtain a continuous variation of the operating pressure P.sub.A prevailing in the larger driving pressure space of the hydraulic cylinder.
In addition to a first pressure outlet where the pressure supply is available a relatively low pressure P.sub.N, according to the present invention, the pressure source also has a second pressure outlet where the pressure supply is provided at a markedly higher pressure level P.sub.H.
The pressure-reversing valve arrangement is controlled by the operating pressure P.sub.A prevailing in the larger driving pressure space of the hydraulic cylinder and which, when and for as long as the operating pressure PA prevails in the larger driving pressure space of the hydraulic cylinder remains smaller than a switching threshold corresponding to a large fraction of, for example, 85% to 95% of the output pressure P.sub.N made available at the low-pressure outlet of the pressure source, will connect the low-pressure outlet to the pressure supply connection follow-up adjusting valve. Alternatively, when and for as long as the operating pressure P.sub.A prevailing in the larger driving pressure space of the hydraulic cylinder remains above the switching threshold, the pressure-reversing valve arrangement will connect the high-pressure outlet of the pressure source to the pressure supply connection of the follow-up adjusting valve.
In accordance with the present invention, the surface-reversing valve is constructed in such a manner that the switching threshold, upon being understepped, triggers the switching back of the surface reversing valve into an operating position associated with the fast operating modes of the hydraulic cylinder that is lower than the switching threshold of the pressure reversing valve.
The pressure switching valve arrangement can be realized by a simple non-return valve that causes the pressure outlets of the pressure source to become cut off.
According to the present invention, the pressure reversing valve arrangement comprises a pressure-controlled 2/2-way valve that, for as long as the operating pressure in the larger driving pressure space of the hydraulic cylinder remains lower than its switching threshold, is maintained in a basic position in which the pressure supply connection of the follow-up adjusting valve is cut off from the high-pressure outlet of the pressure source. Furthermore, when and for as long as the operating pressure in the greater driving pressure space of the hydraulic cylinder is higher than the switching threshold (b.sub.1 .multidot.P.sub.N ;0.5&lt;b.sub.1 &lt;0.95), the pressure reversing valve arrangement switches into an open position in which the high-pressure outlet is connected to the pressure supply connections of the follow-up adjusting valve.
The pressure reversing valve arrangement according to the present invention also includes a non-return valve inserted between the pressure supply connection of the follow-up adjusting valve and the low-pressure outlet of the pressure source, with the non-return valve being maintained in a closed position for as long as the pressure at the pressure supply connection of the follow-up adjusting valve is higher than the output pressure of the low-pressure output of the pressure source.
Advantageously, the 2/2-way valve may constructed as a slide valve and, for the purposes of setting restoring forces in adjusting the switching threshold desired in each particular case, the slide valve may, for example, be provided with an adjustable return spring capable of having its return force set as required.
The pressure-reversing valve, constructed as a slide valve, includes a piston which is adapted to be displaced or pushed into its basic position by a return force of a preset magnitude, with the slide valve including a control end flange that delimits, in a mobile manner, one side of a control pressure space. An area f.sub.2 of a surface of the end flange is dimensioned so that a force that has to be exerted in order to cause the pressure reversing valve to switch into an operating position in which the high pressure outlet of the pressure source is connected to the pressure supply connection of the follow-up adjusting valve requires a pressure P.sub.A determined by the following relationship: EQU P.sub.A .ltoreq.P.sub.N .multidot.b.sub.1,
where:
P.sub.N =pressure supplied by the low pressure outlet means of the pressure source means; PA1 b.sub.1 =a coefficient less than unity, (0.85 &lt;b.sub.1 .ltoreq.0.95, and preferably amounts to 0.95). PA1 b.sub.2 equals a coefficient less than unity (0.85&lt;b.sub.2 &lt;0.95) defining an amount by which the operating pressure P.sub.A at which the seat valve opens may understep a maximum possible operating pressure P.sub.H ; and PA1 a equals a small safety margin of, for example, 2% to 10%.
In order to exploit the lower output pressure of the pressure source in a very simple manner for the purposes of producing the restoring force that is needed for setting the reversing threshold, in accordance with the present invention, the valve piston of the pressure-reversing valve includes another end flange at an end facing away from the control pressure space in which the prevailing operating pressure P.sub.A is the same as the output pressure of the follow-up adjusting valve that is applied to the larger driving pressure space of the hydraulic cylinder. The other end flange forms a mobile delimitation of a control pressure space of the pressure-reversing valve in which there permanently prevails the output pressure P.sub.N available at the low pressure outlet of the pressure source.
The pressure-reversing valve of the present invention is advantageously constructed so no elastic spring elements of any type are necessary for setting the pressure-reversing valve to a desired pressure-setting threshold. Moreover, at least one part that would otherwise be subject to considerable wear and tear also becomes superfluous.
For this purpose, according to the present invention, a ratio f.sub.1 /f.sub.2 between the areas f.sub.1 and f.sub.2 of the end flanges to which are respectively applied the output pressure P.sub.A of the follow-up adjusting valve and the lower output pressure P.sub.N of the pressure source, has the value B.sub.1. Moreover, the valve piston of the pressure-reversing valve is constructed as a free piston.
By dimensioning the working surfaces of the control valve piston and providing a clear cross sectional area of the valve channel of a seat of the valve, upon dually observing such dimensioning, a high reliability of operating control is insured. More particularly, according to the invention, the valve element of the surface cycle reversing valve, which in an open position causes the pressure to become discharged from the smaller driving space of the hydraulic cylinder, is constructed as a non-return valve which, in an opening position, sustains the operating pressure P.sub.A of the smaller driving pressure space of the hydraulic cylinder prevailing in a central valve chamber of the surface cycle reversing valve. A force with which the precompressed valve closing spring pushes a valve body of the non-return valve into a blocking position is equivalent to an opening pressure that corresponds to a large fraction b.sub.2 (0.85&lt;bP2&lt;0.95) of the higher pressure P.sub.H available at the high-pressure outlet of the pressure source.
Advantageously, according to the present invention, the surface-reversing valve includes another valve element fashioned as a slide valve that, for as long as the non-return valve remains in its blocking position, assumes an open position in which the lower output pressure P.sub.N of the pressure source is applied to the smaller driving pressure space of the hydraulic cylinder. Upon an opening of the non-return valve, the other valve switches into its blocking position in which smaller driving pressure space of the hydraulic cylinder becomes cut off from the low-pressure outlet of the pressure source.
When the slide of the additional valve element is constructed as a stepped piston, a weakly precompressed return spring pushes into supporting contact with the valve body of the non-return valve thus maintaining the same in an operating position in which even a displacement of the stepped piston amounting to no more than a small fraction of the opening stroke of the non-return valve or the closing stroke of the slide valve will be quite sufficient to bring the slide valve into a blocking position in which one side of the stepped piston becomes depressurized while a working surface F.sub.5 of its other side, namely, the one that delimits the control pressure space in which there prevails the operating pressure P.sub.A of the larger driving pressure space of the hydraulic cylinder becomes subjected to the pressure P.sub.A. Moreover, the ratio F.sub.4 /F.sub.5 between the control surface F.sub.5 in the cross sectional area F.sub.4 surrounded by a valve seat of the seat valve is such so that within the area F.sub.4 the valve body becomes subjected to the pressure prevailing in the smaller driving pressure space of the hydraulic cylinder for as long as the non-return valve remains in the blocking position, with the ratio F.sub.4 /F.sub.5 satisfying the following relationship: EQU F.sub.4 /F.sub.5 .ltoreq.(b.sub.1 .multidot.P.sub.N +a)/(b.sub.2 .multidot.P.sub.H),
where:
In accordance with still further features of the present invention, the parameter b.sub.1 has a value of between 0.85 and 0.95 and, preferably, close to 0.9; whereas, the parameter b.sub.2 has a value of between 0.8 and 0.95 and, preferably, close to 0.9.
The ratio F.sub.1 /F.sub.3 between the cross-sectional area F.sub.1 of the large driving pressure space of the hydraulic cylinder and the cross-sectional area of the F.sub.3 of the smaller working surface of the hydraulic cylinder is between 1.5 and 3 and, preferably 2.
Advantageously, the larger working surface F.sub.1 of the hydraulic cylinder has an area of between 60 cm.sup.2 and 300 cm.sup.2 and, preferably close to 100 cm.sup.2.
Moreover, a ratio P.sub.A between the high and low output pressures P.sub.H and P.sub.N of the pressure source has a value of between 4 and 2 and, preferably, a value close to 3.
Additionally, the output pressure level at the low-pressure outlet of the pressure source is between 50 bar and 80 bar and preferably, has a value of near 60 bar. The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which shall, for the purposes of illustration only, one embodiment when in accordance with the present invention.